1. Field of the Invention
The present invention relates to communications systems, more particularly, to a communications system using a low cost oscillator and related method thereof.
2. Description of the Prior Art
An area of recent technological advancement concerns communications-positioning systems. One example of this positioning technology is the global positioning system (GPS). The GPS is currently the Earth's only fully-functional satellite navigation system, comprising more than two dozen GPS satellites to broadcast precise timing signals by radio to electronic GPS receivers. These broadcasts allow GPS receivers to accurately determine their location (longitude, latitude, and altitude) in real time, day time, night time, or in any type of weather condition.
Due to the popularity and usefulness of GPS applications, they have also recently begun to be incorporated within communications devices and cellular phones as a single integrated unit. In producing a cost effective cellular phone with GPS functionality, manufacturers have begun sharing common components between the two systems in the integrated device. Particularly, the oscillator (or crystal) is commonly shared due to the high cost associated with each unit. The oscillator provides a clock signal for which circuitry of the device can operate from, and use for the transmission and receiving of signals. FIG. 1 illustrates a shared crystal system as described in the related art, wherein a single oscillator acts as a clock signal source for both a code division multiple access (CDMA) mobile telephone circuit and a GPS navigational circuit.
As illustrated in FIG. 1, a single oscillator 102 is shared between a GPS receiver 100 and CDMA cellular phone circuitry (not shown). The GPS receiver further comprises a low noise amplifier (LNA) 105 for receiving and amplifying an RF signal, mixers 110, 120, 121 and 146 for combining various signals, frequency dividers 130 and 135 for dividing an input frequency, analog to digital converters (ADC) 125 and 126, frequency synthesizer 116, loop filter 145, variable amplifier 112, GPS baseband 114, and voltage controlled oscillator (VCO) 115. Basic functionality of the GPS receiver 100 is well known to a person in the related art, and as such, a detailed description regarding its operation is omitted for brevity.
It is important to note that cellular phone broadcast frequencies differ from GPS broadcasting frequencies. Therefore, in order to share an identical oscillator between cellular phone circuitry and a GPS receiver, the frequency of the oscillator must be processed to match at least one of the components (either GPS or cellular) of the integrated device, or even possibly individually processed from an arbitrary frequency to match both.
The device illustrated in FIG. 1 utilizes an oscillator 102 whose clock signal matches a required frequency for the cellular phone circuitry, but differs from a required system clock frequency for the GPS baseband 114. The clock signal of the oscillator 102 is therefore processed in order to attain a suitable frequency for GPS circuitry application. The VCO 115 generates the GPS system clock signal Z4, whose frequency is contingent upon the voltage input from loop filter 145 further comprising the harmonic frequency oscillator 102, and synthesized signal S74 (fed-back from VCO 115).
An effective GPS receiver however, requires a highly accurate GPS system clock signal Z4 for optimal performance. Since the GPS receiver 100 utilizes a GPS system clock signal Z4 being derived from an oscillator 102 of mismatched frequency, several uncertainties inherently become involved. Also, implementation of a flexible input frequency synthesizer, as shown in FIG. 1 consumes significant power and IC space.
When a common oscillator is shared with a cellular device of a GSM network, and a positioning device (like a GPS navigations system), a different architecture is required. This architecture is used to cope with the need for changing frequencies involved with the changing of GSM base stations. FIG. 2 describes a method to compensate for the use of a single oscillator between a GSM cellular device and a GPS navigations system according to the prior art.
According to FIG. 2, at step 201, a user of a GPS receiver makes a request to receive its position. At step 202, aiding data (approximate location area, time, etc . . . ) is retrieved from either memory or external sources. At step 203, the GPS receiver is initialized to receive signal samples at the initial sampling frequency of the shared oscillator between the devices. The initial frequency adjustment is then recorded in step 204. The receiver is then turned off in step 205 (or 205a). If frequency compensation is required in step 206, the receiver then performs a complex correlation integration using different hypotheses of the frequency offset with the shared oscillator. At step 207, using the compensated integration of step 206, the receiver obtains a pseudo-range of the position of the GPS receiver.
As seen from the description above and FIG. 2, the method 200 can be time consuming and complex. Frequency compensation involves a base hypothesis and algorithm, which may still provide an under optimized resulting frequency and possible discrepancies. The GPS circuitry must also continually reference the GSM cellular circuitry in case of a switch in the baseband frequency. Furthermore, the GPS circuitry must acquire its local clock frequency through internal software using the GSM baseband frequency. This process therefore may also introduce uncertainties in obtaining an accurate GPS system clock for reasons already described above.
Another configuration described in the prior art attempts to overcome some of the deficiencies above by providing a fixed reference clock for the GPS circuitry. This helps ensure that an accurate frequency is utilized for GPS operations to ensure optimal performance while utilizing a synthesizer to generate frequencies for the GSM/CDMA circuitry. However, GSM performance is degraded due to division and synthesis required to obtain a suitable GSM clock signal. Also, phase noise requirements and frequency accuracies are typically not met using this configuration.